support for GNU configure syntax
Correct prefix / CFG_PREFIX work in configure / install.mk
support for GNU configure syntax corrections: patch from brson
auto merge of #9990 : Heather/rust/configure-changes, r=brson

Re-created: https://github.com/mozilla/rust/pull/9760

Now, without adding `pc-linux-gnu` platform

Re-created: #9565

t will be useful for unix systems, some package managers and external tools that is passing / maybe looking for GNU configure syntax options.

One of related bugs: #5138

Also that could be useful in future if project will really install man pages / other stuff using those variables.